Control system for electron beam magnetometer sensor

ABSTRACT

Described is a control system for an electron beam magnetometer sensor of the type adapted to detect disturbances in magnetic field, particularly the magnetic field of the earth, by means of a vacuum tube containing an electron gun which emits an electron beam and directs it between spaced pole pieces located within the envelope of the vacuum tube itself. The earth&#39;&#39;s magnetic field surrounding the tube is concentrated at the gap between the pole pieces such that the presence of a magnetically permeable body near the tube will alter the magnetic field intensity across the gap and cause the electron beam to deflect. The invention resides in the provision of electrical circuitry for sensing this deflection and for indicating the existence of a disturbance and its magnitude, which circuitry is capable of detecting small time varying signals in the presence of a strong static field.

United States Patent Nicholson et al.

[151 3,657,643 1451 Apr. 18, 1972 [54] CONTROL SYSTEM FOR ELECTRON BEAMMAGNETOMETER SENSOR [72] Inventors: James F. Nicholson, Sevema Park;Clarence Williams, Baltimore, both of Md.

[73] Assignee: Westinghouse Electric Corporation, Pittsburgh, Pa.

[22] Filed: Sept. 30, 1969 [21] Appl. No.: 862,375

Electronics; April 3, 1967; pgs. 270- 271.

Primary Examiner-Alfred E. Smith Attorney-4". H. Henson and E, P.Klipfel [5 7] ABSTRACT Described is a control system for an electronbeam magnetometer sensor of the type adapted to detect disturbances inmagnetic field, particularly the magnetic field of the earth, by meansof a vacuum tube containing an electron gun which emits an electron beamand directs it between spaced pole pieces located within the envelope ofthe vacuum tube itself. The earths magnetic field surrounding the tubeis concentrated at the gap between the pole pieces such that thepresence of a magnetically permeable body near the tube will alter themagnetic field intensity across the gap and cause the electron beam todeflect. The invention resides in the provision of electrical circuitryfor sensing this deflection and for indicating the existence of adisturbance and its magnitude, which circuitry is capable of detectingsmall time varying signals in the presence of a strong static field.

4 Claims, 6 Drawing Figures CONTROL c/Rcu r fies 36 INDICATORPATENTEUAPR 18 m2 SHEET 10F QQKQGRIE .EGQB BE 2% 1w v5 TOR: JAMES l-T'M01101. SON 45 CLARENCE mu. mus

PATENTEDAPR 18 I972 SHEET 2 BF 2 INVENTORS. JAMES E NICHOLSON 8 CLARENCEWILLIAMS M 4 W Aflor ney CROSS-REFERENCES TO RELATED APPLICATIONSCopending application Ser. No. 862,376, filed Sept. 30, 1969, andcopending application Ser. No. 862,278, filed Sept.

BACKGROUND or THE INVENTION In the past, magnetometers have been devisedfor detecting disturbances in the earths magnetic field on the principlethat an electron beam, which is centered under the influence of a normalmagnetic field, will be deflected whenever that field is disturbed. Thisdeflection, in turn, can be sensed to actuate an alarm or can be used todrive a meter or recorder which indicates the magnitude of thedeflection.

In copending application Ser. No. 862,376, filed Sept. 30, 1969 andassigned to the assignee of the present application a novel electronbeam magnetometer is described which includes an evacuated envelopecontaining an electron gun adapted to emit a beam of electrons. Thisbeam of electrons is directed against an electron target or collectorcomprising a plurality of segments of a circle separated by a spacetherebetween. In the path of the electron beam between the gun and thetarget are pole pieces separated by a gap through which the electronbeam passes. An external magnetic field, such as the earths magneticfield, is concentrated'across the gap between the pole pieces which arecontained within the evacuated envelope itself. Preferably, theevacuated envelope on either side of the pole pieces is surrounded by ashield of magnetically permeable material which directs the magneticfield across the gap formed between the pole pieces.

When a magnetically permeable object intersects the magnetic field towhich the detecting apparatus is subjected, the field existing acrossthe aforesaid gap between the pole pieces is altered. This alteration,in turn, causes a deflection in the electron beam which can be used toactuate a meter, a

recorder, or an alarm. The invention can be used, for example,

where it is desired to detect concealed weapons. In this case, theearth's magnetic field is that field to which the detecting apparatus issubjected. When an individual carrying a concealed weapon, for example,intersects the magnetic field adjacent the detecting device, theelectron beam is immediately deflected to indicate the existence of theconcealed weapon.

SUMMARY OF THE INVENTION when it is caused to deflect, increases thenegative charge on one of the plurality of sectors forming the target toincrease the charge on one of the vertical or horizontal deflectionplates of the electron gun to again center the electron beam emitted bythe gun.

Still another object of the inventionis to provide a control system ofthe type described which is simple in construction and which not onlyindicates the magnitude of the disturbance in the field sensed by themagnetometer but also actuates an alarm when the magnitude of thedisturbance exceeds a predetermined limit.

In accordance with the invention, apparatus for detecting disturbancesin a magnetic field is-provided comprising an evacuated envelopecontaining an electron gun adapted to emit a beam of electrons. Thisbeamof electrons is directed against an electron target or collector. Inthe path of the electron beam between the gun and the target arepolepieces separated by a gap throughwhich the electron beam passes. Anexternal magnetic field, such as the earths magnetic field,

is concentrated across the gap between the pole pieces which arecontained within the evacuated envelope itself.

The electron target or collector against which" the electron beam isdirected is divided into circular sectors, preferably four in number,which are separated by a space therebetween. In the space between thesectors is a knife-edge comprising a negatively charged plate, which isin a form of a cross in the case of four circular sectors. Thisknife-edge, being negatively charged, will repel electrons, prevent themfrom passing through the space between the sectors, and evenly scatterthe electrons from the beam onto the four sectors when the beam iscentered. However, when a magnetically permeable object intersects themagnetic field to which the detecting apparatus is subjected, themagnetic field existing across the aforesaid gap between the pole piecesis altered. This alteration, in turn, causes a deflection in theelectron beam which unbalances the negative charges on the sectors ofthe target.

In the case of a target comprising four circular sectors, two of thesectors are connected to the horizontal deflection plates of theelectron gun while the other two are connected to the verticaldeflection plates, the arrangement being such that when the electronbeam moves off center in the horizontal direction, for example, thenegative charge on one of the two horizontal deflection plates will beincreased, thereby causing the beam to again move toward center. Theoperation of the other two opposite sectors and the vertical deflectionplates is the same. The horizontal deflection plates are connectedthrough filters and amplifiers to a meter which indicates the magnitudeof the deflection and to an alarm circuit which is actuated whenever thedeflection exceeds a predetennined magnitude, indicating the existenceof a magnetically permeable object in the field above a lower, limitingsize. The filters attenuate all signals above about 10 hertz and, thus,provide efi'ective isolation from noise or other spurious signals.

The above and other objects and features of the invention will becomeapparent from the following detailed description taken in connectionwith the accompanying drawings which form a part of this specification,and in which:

FIG. 1 is a schematic illustration of the magnetometer apparatus of thepresent invention;

FIG. 2 is a cross-sectional view of the magnetometer of the invention;

FIG. 3 is a cross-sectional view taken along line III-III of FIG. 2;

FIG. 4 is a broken-away elevational view of the electron target utilizedin the magnetometer of FIG. 2;

FIG. 5 is an end view of the apparatus shown in FIG. 4; and

FIG. 6 is a schematic circuit diagram of the control apparatus of thepresent invention for the magnetometer shown in FIGS. 2-5.

With reference now to the drawings, and particularly to FIG. 1, themagnetometer of the invention includes an electron-emitting cathode 10and a plurality of focusing grids 12 which direct an electron beam 14between pole pieces 16 and 18 and onto a split target or collector 20which serves the function of an anode in a conventional electron gun.The electron beam 14, after being focused by the grids 12, passesbetween horizontal deflection plates 22 and 24 and vertical deflectionplates 26 and 28. The earth's magneticfield, indicated by the arrows 30in FIG. 1, passes through the upper pole piece 16, thence across gap 32which separates the pole pieces, and then to the pole piece 18. In sodoing, the magnetic field, which is concentrated at the gap 32,intersects the beam 14.

The target 20 is divided into four sectors 20A, 20B, 20C and 20D asshown. These sectors are connected to a control circuit 34, hereinafterdescribed in detail in connection with FIG. 6 of the drawings. Thecontrol circuit 34, in turn, is connected to the horizontal deflectionplates 22 and 24 as well as the vertical deflection plates 26 and 28.

Let us assume, for example, that the electron beam 14 is deflected bythe magnetic field existing acros the gap 32 such that his directed ontothe sector 20A. Under these circumstances, the electron beam has beendeflected to the right as viewed in FIG. 1. The control circuit 34senses the increased negative charge on the sector 20A and, in turn,increases the negative charge on the right horizontal deflection plate24, thereby moving the electron beam back toward the center of thedevice, or to'the left. Similarly, if the electron beam 14 should moveupwardly onto sector 20D, the control circuit 34 will increase thenegative charge on the upper vertical deflection plate 26, therebycausing the electron beam 14 to move downwardly, toward the center. Inthis manner, whenever the electron beam is deflected, it isautomatically recentered by the control circuit 34 and the vertical andhorizontal deflection plates. At the same time, when an unbalancedcondition occurs on the sectors 20A-20D with one sector being chargedmore negatively than the others, this is sensed by the control circuit34 which actuates an indicator or an alarm 36 which indicates theexistence of a magnetically permeable object in the field 30 whichcaused the deflection of the electron beam 14. The control circuit 34and the alarm 36 will hereinafter be described in greater detail.

The mechanical details of an actual magnetometer are shown in FIG. 2. Itcomprises two glass envelopes 38 and 40 interconnected by means of anassembly including a pair of generally annular, threaded members 42 and44. The member 44, for example, has an annular flange 46 welded orotherwise securely fastened to its rear surface. This annular flange 46,in turn, forms a glass-to-metal seal with the glass envelope 40.Likewise, the annular member 42 is provided with an annular flange 50projecting rearwardly which is hermetically sealed to the right end ofthe glass envelope 38.

The tubular members 42 and 44 are interconnected by means of acylindrical spacer 52 which is hermetically sealed to the two members 42and 44. The members 42 and 44 are provided with central openings 54which connect the annular chamber 56 formed by spacer 52 with theinteriors of the glass envelopes 38 and 40. In this manner envelope 38,the chamber 56 formed by the annular spacer 52 and the glass envelope 40form a contiguous enclosure which is evacuated.

Integrally formed with the member 42 is the lower pole piece 18schematically illustrated in FIG. 1. The pole piece 18, as best shown inFIG. 3, forms an approximate segment of a circle. Similarly, the upperpole piece 16 is integrally formed with the member 44 and, in crosssection, is also in the form of an approximate segment of a circle asviewed in FIG. 3. The space between the two pole pieces 16 and 18 formsthe gap 32 referred to with respect to FIG. 1 and through which theelectron beam 14 passes.

The cathode 10, grids l2 and deflection plates 22-28 of FIG. 1,generally indicated by the reference numeral 58 in FIG. 2, are allcontained within the glass envelope 38 connected to the member 42.Connections to these elements are by prongs 59 projecting through theenvelope 38. The split target 20, on the other hand, is contained withinthe glass envelope 40 and is provided with projecting prongs 61 forconnection to external circuitry described in connection with FIG. 6.Electrons emitted by the electron gun 58 will pass through opening 54 ofmember 42, the gap 32, and opening 54 in member 44 to the split target20.

Threaded onto the annular members 42 and 44 are two shields 63 and 65 ofmagnetically permeable material, such as iron. These shields act, ineffect, as collectors of an external magnetic field, indicated by thearrows 30, and direct the field across the gap 32. Thus, the entiremagnetic field surrounding the device is concentrated at the gap 32through which the electron beam passes. All other parts of the devicesuch as the deflection plates and target 20 are essentially unaffectedby the field. A change in the surrounding field due to the entrance of amagnetically permeable object will, in effect, be magnified at the gapto cause the electron beam to deflect with great sensitivity. The.cylindrical spacer 52 is formed from other than magnetically permeablematerial to provide anarea of magnetic isolation, except through thepole pieces.

The details of the target 20 are shown in FIGS. 4 and 5. It comprises apair of cylindrical members 60 and 62 provided with clips 64 embedded ininsulating glass spacers 66 which engage the inner periphery of theglass envelope 40. Intermediate the two cylindrical members 60 and 62 isthe split target 20 itself comprising the four segments 20A-20D. Thesefour segments are provided with clips 68 also embedded in the glassspacer 66. Secured to the rear edge of the annular member 62 isknife-edge 70, in the form of a cross, which extends through the slotsformed between the four quadrants of the collector 20A-20D. Thisknife-edge, in turn, is connected to a source of negative potential asshown in FIG. 1. In actual practice, the knife-edge and sectors shown inFIG. 5 will be tilted at an angle of 45 with respect to the horizontaland vertical deflection plates such that sectors 20A and 20C areresponsive to horizontal excursions of the beam while sectors 20B and20C are responsive tovertical excursions.

The purpose of the knife-edge is to deflect the beam evenly on eitherside of the center of the target or collector assembly 20. If it werenot for the knife-edge, the beam would tend to pass through the openingsformed between the quadrants 20A-20D and the sensitivity of the devicewould suffer. However, with the knife-edge negatively charged, the beamtends to split evenly in all directions. In the absence of deflection ofthe beam 14 due to a magnetic disturbance, the system is balanced withthe beam remaining on-center.

With reference now to FIG. 6, elements shown therein which correspond toelements shown in FIG. 1-5 are identified by like reference numerals.The magnetometer is provided with a pair of input terminals 74 and 76.Terminal 74 is grounded; whereas terminal 76 is connected to a source of300 volts. Connected between the terminals 74 and 76 is a voltagedivider comprising five Zener diodes Z1 through Z5.

The grids 12 are four in number, and identified in FIG. 6 as 12A, 12B,12C and 12D. Grid 12D is connected to the anode of Zener diode Z5 and isat a potential of about 274 volts by virtue of the voltage drop acrossresistor 78. Grids 12A and 12C are both connected to the junction ofZener diodes Z1 and Z2 and are at a potential of about 35 volts.Finally, grid 12B is connected to a movable tap on potentiometer 80connected between the anode and cathode of Zener diode Z4 which are atpotentials of about 250 volts and 2l8 volts, respectively. The cathode10 is connected to the junction of Zener diodes Z4 and Z5 and is at apotential of about 250 volts. Cathode 10 is provided with a heater 82connected to a 6-volt heater supply via terminals 84 and 86.

The horizontal sectors 20A and 20C are connected through resistors 88and 90 to input terminal 74 and, hence, are essentially at groundpotential. Similarly, the vertical sectors 20B and 20D are connectedthrough resistors 92 and 94 to the same input terminal 74. Theknife-edge 70 is connected to the junction of Zener diodes Z3 and Z4 andis at a potential of about 218 volts. As a result, it is at a much lowernegative potential than the sectors 20A-20D and, consequently, scattersthe electron beam evenly onto the sectors, assuming that the beam is notdeflected under the influence of a change in the magnetic field acrossgap 32.

The sectors 20A and 20C are connected directly to the horizontaldeflection plates 24 and 22, respectively. Similarly, the upper andlower sectors 20D and 20B are connected directly to the upper and lowervertical deflection plates 26 and 28, respectively. It might be expectedthat a variation in the magnetic field across the gap 32 would normallycause the electron beam 'to move either to the right or left in thehorizontal direction as viewed in FIG. 1. It has been found,

however, that the beam will move not only to the right or the left butalso upwardly or downwardly; and it is for this reason that the sectors20B and 20D are needed along with the vertical deflection plates 26 and28.

In any event, the beam will diverge to the right or left upon theintroduction of a magnetically permeable body into the field and,accordingly, the sectors 20A and 20C are connected through leads 96 and98 and capacitors 100 and 102 to the gate electrodes of two field effecttransistors 104 and 106. The field effect transistors are the firststages of an indicating circuit. Under balanced conditions, the electronbeam 14 will be equally split between sectors 20A and 20C and betweensectors 20B and 20D. If a field in the gap 32 shown in FIG. 1 acts todeflect the beamonto sector 20A as explained above, this sector goesmore negative; and, thus, horizontal deflection plate 24 which also goesmore negative, deflects the beam back to the center of the tube.

Coupling capacitors 100 and 102 AC couple sectors 20A and 20Crespectively to transistors 104 and 106 respectively to place 'a lowfrequency limit on the operation of the indicating circuit. They preventthe indicating circuit from being a DC system by charging up to amagnitude which is directly related to the voltage on the respectivetarget sectors. When a new magnetic field disturbance occurs, the chargeon the capacitors is changed in order to provide a new indication.

The gate electrode of the field effect transistor 106 is connected tothe junction of resistors 108 and 110 connected between a source ofpositive potential and ground. Similarly, the gate electrode of fieldeffect transistor 104 is connected to the junction of resistors 112 and114 connected between a source of positive potential and ground. Thedrain electrodes of field effect transistors 104 and 106 are connectedto sources of positive potential; whereas their source electrodes areconnected through resistors 116 and 118 to ground.

Thesource electrode of field effect transistor 106 is also connectedthrough capacitor 120 and resistor 122 to ground, the voltage across theresistor 122 being applied through resistor 124 to one input of adifferential amplifier 126. Similarly, the source electrode of fieldeffect transistor 104 is connected through capacitor 128 and resistor130 to ground, the voltage appearing across resistor 130 being appliedthrough resistor 132 to the other input of the differential amplifier126. The amplifier 126 is provided with a feedback loop including aresistor 134 in shunt with capacitor 136. The capacitor 136, having avalue of about 0.056 microfarad, acts as a filter which attenuatessignals having a frequency above hertz. In this respect, the impedanceof the capacitor 136 drops practically to zero when the frequencyexceeds 10 hertz and, thereby, shorts out the resistor 134; With thisarrange ment, extremely good isolation from noise signals above 10 hertzis provided.

The output of the differential amplifier 126, comprising a signalproportional to the difference between thepotentials on sectors A and20C, is applied through couplingcapacitors 138 and 140 across resistor142. The voltage across resistor 142 is applied through resistor 144 toone input of a second amplifier 146, the other input of which isconnected to ground through resistor 148. Amplifier 146, like amplifier126, is provided with a feedback path including resistor 150 connectedin shunt with capacitor 152. The capacitor 152, like capacitor 136, actsas a rough lO-hertz filter for signals passing through the amplifier.The two amplifiers 126 and 146 are employed rather than a singleamplifier in order to divide the gain required between the twoamplifiers.

The output of the amplifier 146 is applied through resistor 154 to aIO-hertz filter comprising an operational amplifier 156 having one ofits input terminals connected to ground through resistor 158 andcapacitors 160 and 162. This same input terminal is connected throughcapacitor 164 and resistor 166 to one end of resistor 154; and is alsoconnected through capacitor 164 to the output of the operationalamplifier 156. The other input terminal of the operational amplifier 156is grounded as shown. The lO-hertz filterthus provides gain as well asattenuation of signals at a frequency above lO-hertz.

The output of the operational amplifier 156 is applied through resistor168 and rectifiers 170 and 172 to a meter 174 which indicates themagnitude of the pulses at the output of amplifier 146. These samepulses are applied through capacitors 176 and 177 across resistor 180.The voltage across resistor 180 is then rectified in a full-waverectifier comprising operational amplifier 182, one terminal of which isconnected through a resistor 184 to the capacitor 176, and the otherinput terminal of which is connected to the junction of diodes 186 and188 connected between one end of the resistor 184 and ground. The upperinput terminal of the operational amplifier 182 is also connectedthrough resistors 190 and 192 to its output.

At the output of the full-wave rectifier is a clipping diode 194 havingits cathode connected through resistor 196 to a potentiometer 198 havingone terminal connected to a source of +12 volts and its other terminalconnected through resistor 200 to a source of l2 volts, as well asthrough diode 202 to ground. By adjusting the potential on the cathodeof diode 194, the bottom portion of the pulses at the output of theamplifier 182 can be clipped. These pulses are applied through capacitor204 and a resistor 206 to one input of an operational amplifier 208which acts as an isolation amplifier. The output of the isolationamplifier, comprising the clipped pulses, is then applied throughcapacitor 210 to a direct current restoring network including resistors212 and 214 connected to a source of +12 volts. The junction ofresistors 212 and 214 is connected to the junction of two diodes 216 and218 connected between the upper terminal of resistor 212 and ground. Thedirect current restoring circuit is needed since the bottom portions ofthe pulses have been clipped and, therefore, it is necessary toestablish a direct current reference potential.

The restored pulses are then applied through diode 220, which acts as anon-linear resistance element, and resistor 222 to one input of anoperational amplifier 224, the other input of which is connected throughresistor 226 to ground. As the input pulses go more positive, the gainof the amplifier 224 increases. That is, the gain is a function of theamplitude of the input signal. One input of the operational amplifier isconnected to its output through resistor 228 as shown.

The output of operational amplifier 224 is connected through resistor230 to one input of a comparator operational amplifier 232 whichcompares the amplitude of the pulses at the output of amplifier 224 witha reference potential established by resistors 234 and 236 along withdiode 238. Assuming that the amplitude of the pulse at the output ofoperational amplifier 224 exceeds the reference potential applied to theother input terminal of the operational amplifier232, an output isproduced which is applied through resistor 240' to the base oftransistor 242'to energize a lamp 244. As will be understood, other anddifierent types of alarms can be used to indicate the existence of amagnetically permeable object above a given mass in the field to whichthe magnetometer is subjected.

Although the invention has been shown in connection with a certainspecific embodiment, it will be readily apparent to those skilled in theart that various changes in form and arrangement of parts may be made tosuit requirements without departing from the spirit and scope of theinvention. In this respect, it will be apparent that vertical andhorizontal deflection coils, rather than deflection plates can be usedin the invention with equal effectiveness.

We claim as our invention:

1. In apparatus for detecting disturbances in a magnetic field caused byimposition of a magnetically permeable member in said field, thecombination 'of an evacuated envelope containing an electron emittingcathode and a plurality of focusing grids through which a beam ofelectrons emitted by said cathode passes, at least one pair ofoppositely disposed beam deflection devices within said envelope, a pairof pole pieces separated by a gap through which said electron beampasses, said pole pieces in cross section defining oppositelydisposedsegments of a circle with said gap being between the radially innermostends of said segments, means for causing said magnetic field to passacross said gap, a target against which said electron beam is directedafter passing between said pole pieces, said target being formed from atleast two separated parts, means electrically connecting one of saidparts to one of said oppositely disposed beam deflection devices, meansconnecting the other of said parts to the other of said oppositelydisposed beam deflection devices, the arrangement being such that whenthe beam is deflected from'a central position the negative charge on oneof said parts will be increased to cause the beam deflection device towhich it is connected to again center said beam, differential amplifiermeans connected to the respective parts of said target and adapted toproduce an output signal equal to the difference between the charges onthe respective parts of the target, operational amplifier means andrectifying means coupled to the output of said differential amplifiermeans for producing a direct current signal proportional in magnitude tothe difference output of said differential amplifier means, meter meanscoupled .to said rectifying means for indicating the magnitude of saiddirect current signal, and alarm means coupled to said operationalamplifier means for indicating deflection of said beam above apredetennined amount.

2. The apparatus of claim 1 wherein said beam deflection devicescomprise electrostatic deflection plates connected directly to therespective parts of said target.

3. The apparatus of claim 1 wherein said beam deflection devicescomprise horizontal electrostatic deflection plates and verticalelectrostatic deflection plates, said target being formed from fourparts, two oppositely disposed parts of said target being connected tothe respective horizontal deflection plates and the other two oppositelydisposed parts being connected to said vertical deflection plates.

4. The apparatus of claim 2 including capacitor means connected to therespective parts of said target for storing at least a portion of theelectrical charge on said parts.

1. In apparatus for detecting disturbances in a magnetic field caused byimposition of a magnetically permeable member in said field, thecombination of an evacuated envelope containing an electron emittingcathode and a plurality of focusing grids through which a beam ofelectrons emitted by said cathode passes, at least one pair ofoppositely disposed beam deflection devices within said envelope, a pairof pole pieces separated by a gap through which said electron beampasses, said pole pieces in cross section defining oppositely-disposedsegments of a circle with said gap being between the radially innermostends of said segments, means for causing said magnetic field to passacross said gap, a target against which said electron beam is directedafter passing between said pole pieces, said target being formed from atleast two separated parts, means electrically connecting one of saidparts to one of said oppositely disposed beam deflection devices, meansconnecting the other of said parts to the other of said oppositelydisposed beam deflection devices, the arrangement being such that whenthe beam is deflected from a central position the negative charge on oneof said parts will be increased to cause the beam deflection device towhich it is connected to again center said beam, differential amplifiermeans connected to the respective parts of said target and adapted toproduce an output signal equal to the difference between the charges onthe respective parts of the target, operational amplifier means andrectifying means coupled to the output of said differential amplifiermeans for producing a direct current signal proportional in magnitude tothe difference output of said differential amplifier means, meter meanscoupled to said rectifying means for indicating the magnitude of saiddirect current signal, and alarm means coupled to said operationAlamplifier means for indicating deflection of said beam above apredetermined amount.
 2. The apparatus of claim 1 wherein said beamdeflection devices comprise electrostatic deflection plates connecteddirectly to the respective parts of said target.
 3. The apparatus ofclaim 1 wherein said beam deflection devices comprise horizontalelectrostatic deflection plates and vertical electrostatic deflectionplates, said target being formed from four parts, two oppositelydisposed parts of said target being connected to the respectivehorizontal deflection plates and the other two oppositely disposed partsbeing connected to said vertical deflection plates.
 4. The apparatus ofclaim 2 including capacitor means connected to the respective parts ofsaid target for storing at least a portion of the electrical charge onsaid parts.